The present invention related to an optical recording medium, and particularly to an optical recording medium which is able to record and/or reproduce while using a plurality of laser beam wavelengths as well as a method of recording and/or reproducing information in which such an optical recording medium is used.
Recently, an optical recording medium (such as CD-R, CD-RW (re-writable) and the like) of a single substrate type has been proposed and developed which is capable of recording information (including recording other information in addition to already recorded information) according to the specification for compact disc (hereinafter referred to as CD) and which has a high beam reflection ratio (or reflectance).
As one example of the above optical recording mediums, CD-R will be explained below. As schematically shown in FIG. 4 which shows a portion of a cross section perpendicular to the disc surface, this optical recording medium comprises a transparent substrate 1 having grooves 6 on which a recording layer 2, a reflection layer 3 and a protection layer 4 are laminated in the listed order. A reflectance at the groove 6 is not less than 65% before recording, which is reduced by a laser beam having a high power (i.e. a recording beam) to form pits since the recording layer has been changed physically and/or chemically. Thus formed pits have the reduced reflectances, so that portions having such different reflectances resulted with the irradiated laser beam make it possible to record information. Also, reproduction of the information can be carried out by irradiating a laser beam having a less power onto portions of the optical recording medium and detecting reflectance changes.
For the recording/reproducing with the optical recording medium, a laser beam having a wavelength in the range between 770 nm and 830 nm is generally used. However, a laser beam having a wavelength in the range between 620 nm and 690 nm is used for DVD which has been recently available as an optical recording medium having a larger capacity. Reflectances of the mediums conventionally used for CD for AV (audio-visual) or CD-ROM for computers are less dependent on the laser beam wavelengths so that those mediums are readily used for reproduction by DVD players.
On the other hand, since pigments (or dyes) are used in the conventional CD-R mediums, the reflectances of the mediums are more dependent on the laser beam wavelengths: The reflectance is not less than 65% at a laser beam wavelength of around 780 nm. However, since the pigments used for the recording layer absorb a larger amount of beam having a wavelength in the range between about 620 nm and 690 nm, the reflectance for such a beam is as small as around 10%. Therefore, it is very difficult to reproduce information which is recorded in a CD-R according to the CD specification using a DVD player. In order to overcome the above interchangeability problem (i.e. that the DVD player is not able to reproduce the information recorded according to the CD specification), there have been attempts to provide a plurality of enhancing layers having different optical constants of inorganic materials (such as SiO2) between the substrate and the recording layer, so that the reflectance at a laser beam wavelength in the range between about 620 nm and 690 nm is improved by means of an interference effect.
However, it is not necessarily easy to merely use the enhancing layers having the different optical constants as described to sufficiently improve the reflectance. Further, even though the reflectance is improved by some degree, no sufficient modulation is obtained, and recording sensitivity is lowered so that signal qualities are not satisfactory. Due to the reasons as described, it has been very difficult to provide an optical recording medium having interchangeability between the CD player and the DVD player (that is, an optical recording medium form which the recorded information is able to be reproduced using either player).
In order to achieve the object as described above, the present invention provides an optical medium comprising a transparent substrate which contains thereon at least a recording layer, a recording supplemental (or auxiliary) layer, a reflection layer and a protection layer, wherein: the recording layer and the recording supplement layer contain organic pigments as their main components; the optical recording medium is able to record and/or reproduce using a laser beam having a wavelength of xcex1, and also able to record and/or reproduce using a laser beam having a wavelength of xcex2 which is shorter than xcex1; the recording layer has a spectral (or spectroscopic) absorbance in the range between 5% and 35% at a wavelength of xcex1 and a spectral absorbance in the range not less than 15% at a wavelength of xcex2; and the recording supplemental layer has a peak of a spectral reflectance at a wavelength in the vicinity of xcex2.
the recording supplemental layer has a peak of a spectral reflectance at a wavelength in the vicinity of xcex2.
With the optical recording medium according to the present invention, it is preferable that the spectral (or spectroscopic) reflectance measured through the substrate varies with a wavelength of the beam, and that the spectral reflectance shows a maximum peak at a wavelength in the vicinity of xcex2, and as the wavelength is increased from xcex2 thereafter, and the spectral reflectance is decreased and shows a minimum peak at a wavelength between xcex1 and xcex2 followed by being increased.
It is to be noted that the optical recording medium according to the present invention may be in any of the following embodiments:
(1) An embodiment in which recording/reproducing by means of a laser beam having a wavelength of xcex1 as well as recording/reproducing by means of a laser beam having a wavelength of xcex2 is possible;
(2) An embodiment in which recording by means of a laser beam having a wavelength of xcex1 as well as reproducing by means of a laser beam having a wavelength of xcex2 is possible; and
(3) An embodiment in which recording by means of a laser beam having a wavelength of xcex2 as well as reproducing by means of a laser beam having a wavelength of xcex1 is possible. That is, with the optical recording medium according to the present invention, both of recording and reproducing are possible with respect to two kinds of wavelengths xcex1 and xcex2, and alternatively recording is possible with respect to one wavelength of xcex1 and xcex2 and reproducing is possible with respect to the other wavelength of xcex2 and xcex1. In any embodiment, the optical recording medium according to the present invention may be referred to as an xe2x80x9coptical recording medium applicable to two kinds of wavelengthsxe2x80x9d since it is used with respect to the two kinds of wavelengths.